The present invention relates generally to medical treatments. More specifically, the present invention relates to pressure sensing devices for medical fluids.
Due to disease, insult or injury, a person may require the infusion of a medical fluid. It is known to infuse blood, medicaments, nutrients, replacement solutions, dialysis fluids and other liquids into a patient. It is also known to remove fluid from a patient, for example, during dialysis. Dialysis is used to treat renal system failure, including kidney failure and reduced kidney function.
Renal failure causes several physiological effects. The balance of water, minerals and the excretion of daily metabolic load is no longer possible in renal failure. During renal failure, toxic end products of nitrogen metabolism (urea, creatinine, uric acid, and others) can accumulate in blood and tissues. Dialysis removes waste, toxins and excess water from the body that would otherwise have been removed by normal functioning kidneys.
Hemodialysis and peritoneal dialysis are two types of dialysis therapies commonly used to treat loss of kidney function. Hemodialysis (“HD”) treatment utilizes the patient's blood to remove waste, toxins and excess water from the patient. The patient is connected to an HD machine and the patient's blood is pumped through the machine. Catheters are inserted into the patient's veins and arteries to connect the blood flow to and from the HD machine. As blood passes through a dialyzer in the HD machine, the dialyzer removes the waste, toxins and excess water from the patient's blood and returns the blood to infuse back into the patient.
Peritoneal dialysis (“PD”) utilizes a dialysis solution or “dialysate”, which is infused into a patient's peritoneal cavity. The dialysate contacts the patient's peritoneal membrane in the peritoneal cavity. Waste, toxins and excess water pass from the patient's bloodstream through the peritoneal membrane and into the dialysate. The transfer of waste, toxins, and water from the bloodstream into the dialysate occurs due to diffusion and osmosis, i.e., an osmotic gradient occurs across the membrane. The spent dialysate drains from the patient's peritoneal cavity and removes the waste, toxins and excess water from the patient. This cycle is repeated on a semi-continuous or continuous basis. There are manual PD techniques, known as Continuous Ambulatory Peritoneal Dialysis (“CAPD”). There are also Automated Peritoneal Dialysis techniques (“APD”).
In each type of dialysis treatment, it is critical to know the pressure of the fluid that is being transported to or from the patient. Moreover, in any type of blood transfusion, saline transfusion, or any other type of fluid infusion or flow to or from a patient's body, it is important to know and control the pressure of fluid entering and leaving a patient's body.
Fluid pressure, generally, is sensed using a transducer or strain gauge. Medical fluid transducers have included strain gauges made from a silicon chip. Some medical fluid pressure transducers employ a mechanical linkage to transmit pressure from the fluid to the strain gauge. Many medical transducers, however, have abandoned the mechanical linkage in favor of a hydraulic pressure coupling medium comprised of a silicone elastomer, or “silicone gels”. In use, the gel is positioned between the medical fluid (that is sensed for pressure) and the transducer chip, wherein the gel conveys a hydraulic pressure signal to the integral sensing diaphragm of the transducer chip. At the same time, the gel isolates the chip electrically from the medical fluid.
In one type of medical transducer, the entire transducer assembly, including the chip, is discarded after a single use, since the internal components cannot be adequately cleaned for resterilization or reuse. Disposable transducer designs employing semiconductor strain gauge sensors and gel coupling media are desirable because they are rugged and accurate. Further, the disposable transducers do not require attachment of a separate disposable dome as do reusable types of medical pressure transducers.
Regardless of the advantages of the completely disposable medical pressure transducers, manufacturing costs for the pre-calibrated semiconductor chip and associated wiring of these types of transducers remain high. Moreover, the electronics, which could otherwise be reused, are thrown away with the rest of the unit. This is wasteful and costly. Indeed, because the waste contains electronics, it is more costly to dispose.
Accordingly, a pressure sensor that enables the valuable electronics of the transducer to be reused and allows the inexpensive sterile portion for the transfer of the medical fluid to be discarded is desirable. Such pressure sensors exist and typically have a dome portion, which defines a fluid lumen for the medical fluid, and a transducer portion, housing the electronics. The hurdle presented by these types of sensors is in trying to accurately transfer pressure fluctuations in the dome to like fluctuations in the transducer.
In many systems, the medical fluid carrying dome employs a first membrane and the transducer employs a second membrane. The two membranes abut one another and attempt to transmit medical fluid pressure fluctuations through to the strain gauge. One problem with these sensors that employ a membrane to membrane seal is in attempting to maintain the seal along the length of the membranes. A slight amount of air entering even a small part of the interface between the two membranes can falsify readings.
A similar problem exists with materials that have been used for the membranes. In particular, dome membranes can be susceptible to gas diffusion. Certain materials, such as ethylene propylene diene methylene (“EPDM”), have relatively high vapor transmission properties, enabling gas to diffuse from the dome, through the dome membrane, and into the interface between the membranes.
A need therefore exists for a medical fluid pressure sensor having a reusable transducer, a disposable medical fluid dome and an improved and repeatable seal between abutting membranes.